Older, commonly employed temperature control systems utilize a passive, non-powered thermostat which typically includes a bimetal element or coil which senses the temperature within a space and which coacts with an electrical contact coupled to a two-wire control line for controlling only the heating function. A manual lever is provided for setting the desired temperature to be maintained and changes in the length of the bimetal element will cause the contact to either separate or electrically connect the wires comprising the control line, thereby energizing or disabling the controller which operates the heating system.
More recently and with the advent of small, economical digital circuit components including microcomputer integrated chips, means have been devised whereby such thermostats, requiring electrical power for their operation, may provide computational and control functions for permitting significant energy conservation. In particular, such thermostats enable the user to program the device to maintain a lower, energy-conserving temperature within the space during those hours or days when the occupants are absent or sleeping and a higher, more comfortable temperature at other times. Auxiliary or backup batteries are frequently employed in such thermostats to retain volatile memory in the event of a failure of the main power source. The inconvenience of re-programming is thereby avoided.
Thermostats of this type are coupled to a multiple wire control line which supplies power to the thermostat and which provides the interconnection means whereby the thermostat selectively operates the heating or cooling system controller. Typically, the system including the multiple wire control line is configured such that the thermostat must derive its operating power from the device being controlled through those same wires to which it is connected for performing control functions. An example of a thermostat which derives its power from the device being controlled is shown and described in U.S. Pat. No. 4,274,145. Another example of a thermostat which derives its power from the heating system being controlled and which includes a back up battery for microcomputer memory retention is described in U.S. Pat. No. 4,257,555.
With the advent of the triac, a solid state power switching device, product designers have recognized that such triacs may advantageously be incorporated within microcomputer based thermostats, within heating/cooling system controllers or both. It is known that a triac requires a predetermined minimum level of current flowing therethrough in order to maintain the triac in a conductive state. If switching triacs are employed only in the thermostat and if the thermostat derives its power from those same wires used to operate the system controller, care must be taken to assure that the controller connected thereto requires an operating current sufficiently high to maintain the triacs in a conducting state at those times dictated by the thermostat. Conversely, if triacs are employed only in the controller connected to the thermostat by wiring which is common to both power and control functions, design precautions must be taken to assure that the scavenging of power by the thermostat from the control line will not cause a malfunction in the controller. If triacs are used in both the thermostat and the controller, special provisions must be made to precisely, simultaneously control the switching points of the two sets of triacs, a difficult and relatively expensive control feat.
One approach to the design problem presented by the evolution of new solid state and integrated chip circuit components is shown and described in U.S. Pat. No. 4,298,946. The thermostat shown and described therein utilizes switched triacs to drive electromagnetic relays, the latter being used to selectively activate the heating or cooling system controller. These relays render a thermostat of this type insensitive to the electrical load characteristics of the controller since, insofar as the controller is concerned, the relays present one or more sets of mechanical contacts which are purely passive. However, a thermostat of this type must nevertheless derive its primary operating power from a separate alternating current source and must incorporate an additional power source, a backup battery, for retaining the digital microcomputer memory. Yet another approach to this design problem is shown in U.S. Pat. No. 4,300,199 which similarly uses triacs to drive output relays, but yet derives its operating power from either the heating or cooling unit to which it is connected or from a separate transformer.
While these thermostats have heretofore been generally satisfactory, they tend to be characterized by certain disadvantages. In particular, they are required to derive their operating power either from the device being controlled or from separate, dedicated AC power sources and additionally utilize batteries solely for the retention of microcomputer memory in the event of a power failure. A thermostat which utilizes an extremely low power microcomputer, which derives its operating and memory retention power solely from a battery, which incorporates an isolating interface device for rendering the thermostat and the load controller adapted to be coupled thereto insensitive to the electrical characteristics one to the other and which may be used to provide load-insensitive control by a control line having two or more wires would be a significant advance in the art.